Harvesting energy from sunlight to produce electricity using photovoltaic devices provides a promising way to produce a clean and renewable source of energy. During the past decade, a significant amount of effort has been devoted to the development of polymer-based solar cells. Polymeric materials offer unique advantages over inorganic materials such as low-cost processability and flexibility. The more efficient organic solar cells are often based on a bulk heterojunction (BHJ) structure were the interface between the donor and acceptor provides an efficient charge separation leading to high photocurrents. Polymer bulk heterojunction (BHJ) solar cells offer a compelling option for tomorrow's photovoltaic devices since they can be easily prepared using low-cost and energy efficient roll-to-roll manufacturing processes. Although BHJ solar cells have made great progress over the last several years with power conversion efficiencies reaching over 6%, higher efficiency and stability are desired for large-scale production and commercialization of photovoltaic devices. Low bandgap polymers were expected to harvest more photons and improve the power conversion efficiency of organic solar cells.
Bulk heterojunction solar cells based on a regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester ([60]PCBM) blend have been widely investigated [1-3]. However, BHJ devices made from such low bandgap materials, using PCBM (C60) as acceptor, are usually not highly efficient. Most of them suffer from mismatches between HOMO-LUMO energy levels, low hole mobility, and low open circuit voltage (Voc) all of which lead to low short circuit currents (Jsc) and a small fill factor (FF). Power conversion efficiencies (PCE) up to 5-6% have been reported. The relatively large band gap of 1.9 eV and a HOMO energy level of 5.1 eV prevent P3HT/PCBM-based BHJ solar cells to reach higher PCE values.
To achieve higher power conversion efficiencies, a good balance of the bandgap and energy levels of both donor and acceptor materials to enhance the Voc and the Jsc are required. The Voc is typically defined by the difference between the HOMO energy level of the electron donor (polymer or small molecules) and the LUMO energy level of the electron acceptor (most often [60]PCBM). To achieve high PCEs using the BHJ configuration, the ideal electron donor should have a bandgap ranging between 1.2 and 1.9 eV, a HOMO energy level ranging between −5.2 and −5.8 eV and a LUMO energy level ranging between −3.7 and 4.0 eV. These properties will promote efficient charge separation and maximize the open circuit potential (Voc).
The past few years have witnessed the development of several new classes of conjugated polymers that have been used as electron donors in BHJ solar cells [4-6]. Lately, power conversion efficiencies up to 8.1% have been reported confirming that organic photovoltaic technology can become a cost effective and competitive technology.
Donor-Acceptor (D-A) structures have been widely used to reduce the bandgap of polymers. 2,1,3-Benzothiadiazole (BT) or 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT) have been synthesized and copolymerized with electron-donor co-monomers such as fluorene, dibenzosilole, carbazole, dithienole, and cyclopenta[2,1-b:3,4-b]dithiophene leading to PCEs of up to 6%. However, only a few good electron-acceptor structures have been reported in the literature when compared to electron-donor units.
Tour et al reported an imido-containing polythiophene with reduced optical bandgap [7, 8]. Pomerantz et al performed ab initio calculations on thieno[3,4-c]pyrrole-4,6-dione (TPD) which revealed that polythiophenes with carbonyl groups in both the 3- and 4-positions are planar. Planarity is the result of coulombic attraction between the carbonyl oxygens and the sulfur atom in the adjacent ring [9, 10]. Bjornholm et al. reported a detailed synthesis of homopolymers based on thieno[3,4-e]pyrrole-4,6-dione [11]. The use of conjugated thiophene-comprising polymers as organic electrodes has been described in International publication WO 2008/144756 [12].
The TPD structural unit represents an attractive building block since it can be readily prepared from commercially available starting materials. Moreover, it exhibits a compact planar structure which is beneficial to electron delocalization when incorporated into various conjugated polymers. Furthermore, its planar structure is beneficial in promoting intra- and inter-chain interactions along and between coplanar polymer chains, while its strong electron withdrawing effect leads to lower HOMO and LUMO energy levels, a desired property for increasing the stability and the Voc in BHJ solar cells. Copolymers based on benzodithiophene (BDT) and TPD were recently reported. A power conversion energy of 5.5% was obtained for a PBDTTPD/[70]PCBM blend having an active area of 100 mm2 [13]. It was subsequently reported that copolymers based on BDT and the TPD unit can reach higher power conversion efficiencies. Jen et al. have reported a power conversion efficiency of 4.1% for a PBDTTPD/[70]PCBM (ratio 1:2) blend while Fréchet et al. and Xie et al. have reported power conversion efficiencies ranging from 4.0% to 6.8% for a series of alkylated TPD-based copolymers [14-16]. Lately, Wei et al. have reported a power conversion efficiency of 4.7% and a high Voc of 0.95V using a copolymer based on TBD and bithiophene derivatives [17].
The present specification refers to a number of documents, the content of which is herein incorporated in their entirety.